Obviously It Is Bad Luck To Get Cancer

Let me just cut to the tl;dr: yeah, it still totally matters whether or not you smoke.

There’s a new study on cancer risk that was published this week in Science. It does not in fact say that the majority of cancer cases are the result of bad luck — though that’s how C|Net reported it.

A new study, published in this month’s issue of Science magazine, offers the painful conclusion that lifestyle factors — often these attributed as a cause of many cancers — only account for around one-third of cases.

What the study actually says is that two-thirds of the types of cancer that they analyzed in the study are generally the result of bad luck rather than risk factors. They helpfully divided the cancers in the study into categories they called “deterministic” and “replicative.” Deterministic tumors were caused by genetics and environmental exposures (cigarette smoke, sunshine), while “replicative” tumors were more likely to be random.

So, here’s the thing. About 500,000 people die of cancer every year in the U.S. Of those people, 40,450 died from breast cancer; 50,310 died of colorectal cancer; 159,260 died of lung cancer; and 29,480 died of prostate cancer. That covers well over half the cancer deaths just with those four cancers. And lung cancer, if you’re a smoker, is filed under “deterministic.” Colorectal cancer, if you happened to have one of the genes for it, is filed under “deterministic.” And they didn’t look at breast cancer or prostate cancer at all, but breast cancer is definitely genetically influenced and so is prostate cancer (even if they can’t test you yet for the gene). If we’re going to look at cases rather than deaths, well, 2.8 million people a year get basal cell carcinoma, which was also solidly in the “yep, something definitely caused this” category.

Essentially what the scientists here tried to answer was this question: does it look like the cancer risk for a particular body part is tied to the lifetime number of stem cell divisions for that body part? (“This concept predicts that there should be a strong, quantitative correlation between the lifetime number of divisions among a particular class of cells within each organ (stem cells) and the lifetime risk of cancer arising in that organ.”) The answer was a solid yes. This helps to explain why certain types of cancer are more common than others. The study notes, “cancers of the small intestinal epithelium are three times less common than brain tumors, even though small intestinal epithelial cells are exposed to much higher levels of environmental mutagens than are cells within the brain” — so, basically, the small intestinal epithelial cells undergo fewer stem cell divisions than the cells within the brain, and thus have fewer opportunities to introduce problems that might later lead to malignant tumors.

If that all made your eyes glaze over, I have a helpful metaphor. Imagine a dice game. You get a cup full of dice to roll. (We’ll make them six-sided dice, for the benefit of the readers here who never played D&D.) If you roll your dice and they all come up ones, you get cancer. There are a bunch of dice in your cup, so on any given throw, your odds of rolling all 1s are low.

What this study is showing: with some of your body parts, you roll those dice way more times, and thus have way more opportunities to get that line of all ones.

Here’s the other thing they are showing: with some body parts, you just get a cup of dice and that’s it. It’s way more about “how often do you roll those suckers” than anything else. With other body parts, there are environmental exposures that take away some of your dice, making it easier to get that hand of all ones. Or, your genetics could give you a smaller cup of dice from the get-go.

Anyway. The focus of the study was on how often each body part rolls that metaphorical cup of dice. It shouldn’t be surprising that the more times you roll, the more likely you are to turn up a hand of all ones.

Lost in the “ha, turns out it’s all luck!” misinformation were two bits of data I found extremely surprising. In the “replicative” (random) category were ovarian cancer — which is one of the cancers you’re at risk for if you carry the BRCA mutations, so it’s supposed to be heavily genetic! — and melanoma, which we’re supposed to be slathering ourselves in sunscreen to avoid. The melanoma-UV exposure connection appears to be somewhat less straightforward than the sunscreen ads would have you believe. Although, apparently it is also heavily genetic. They were classifying most of the genetic cancers as “deterministic” (even though a lay person might reasonably consider it bad luck to inherit BRCA1, for instance.)

The misunderstanding of this study likely spread as rapidly as it did because it reinforced a belief that people find either horrifying or weirdly reassuring – or possibly both. Some people want to believe we can control our fate and if they abstain from cigarettes, fried foods, diet sodas, and mixed drinks, they will be exempt from fates like cancer. Others — even a fair number of non-smoking celery-eating hand-washing exercise devotees — would prefer that whatever unhealthy choices they make along the line not result in punishment. Very likely some of the social media traction was the result of people wanting to rub their celery-eating sister-in-law’s nose in the idea that her healthy choices wouldn’t protect her.

We all know people who were just randomly unlucky. And, in fact, most cancer deaths are the result of factors outside our control. Our genetics were determined for us. Our race, sex, and age are not things we get to pick. There are a host of environmental exposures that might increase our risks, but in some cases we don’t know what they are, in other cases we found out too late about the risk, and in still others, we can’t opt out unless we want to move to Nunavut or Namibia.

The useful take-away from the circulating misinformation: don’t assume that people got cancer because of something they did. Don’t assume that living a virtuous life will keep you healthy. But yeah, don’t take up smoking. Even if you never roll snake eyes on the lung cancer, heart disease and COPD are waiting in the wings.